A gasoline engine generates power by burning a fuel/air mixture in a combustion chamber. Such a gasoline engine is equipped with a throttle valve for controlling the air drawn into the combustion chamber. The throttle valve is closed in an idle state, a state where the engine load is low, such that a minimal amount of air is inducted into the combustion chamber. In idle, the engine load varies based on a variety of factors such as whether the air conditioner is operated, and how hot the engine coolant is.
Therefore, a gasoline engine is usually provided with an idle speed controller (ISC) for stabilizing the idle speed when the idling load varies. The ISC includes an idle speed actuator (ISA) for controlling an amount of air bypassing the throttle valve. An engine control unit (ECU) activates the ISA to control the engine output power in idle.
When the throttle valve is abruptly shut after being wide open, the amount of air inducted into the engine decreases abruptly. Therefore, the engine speed is apt to become temporarily lower than a target idle speed. In some extreme cases, the engine may stall. A conventional solution is to provide a mechanical dashpot or a dashpot function for controlling the ISA. FIG. 1 illustrates a dashpot function, where the horizontal axis denotes time, and the vertical axes respectively denote: (1) in the middle, a throttle valve opening; (2) in the top, an engine speed according to the throttle opening; and (3) in the bottom, the amount of air drawn into the engine by ISA control based on the throttle valve operation.
As shown in FIG. 1, when the throttle valve opening is abruptly increased at an instant t0, the engine speed rapidly increases. In this case, an engine control unit controls the ISA to a duty ratio such that the ISC air, the air bypassing the throttle valve, greatly increases to a target ISC amount.
When the throttle valve is closed at an instant t1, the engine speed starts to rapidly decrease. The ISA is maintained open for a delay time DLY_TM (DLY_TM=t2−tl. After the delay time DLY_TM has elapsed at t2, the ISA is controlled so the ISC air monotonically decreases. Therefore, the engine speed rapidly decreases during the delay time DLY_TM. The engine speed then more gradually converges to a target idle speed while the ISC air is monotonically decreasing (after t2). The target ISC amount is calculated from a map table based on a coolant temperature T and a throttle valve opening TH. The delay time DLY_TM is calculated from a map table based on the coolant temperature T.
The dashpot function is implemented using an open-loop control method; the engine speed is not fed back to close the loop. The engine speed decrease is only produced by the delay time DLY_TM and the ISA control pattern. But an engine speed behavior may vary based on the engine operating conditions and/or fuel combustion state. When fuel is cut off while the vehicle is being driven, or when shifting to neutral, the engine speed may rapidly drop to an undesirably low speed or hang at a high speed depending on the exterior air temperature, the air/fuel ratio, and/or the engine age, as well as coolant temperature and throttle valve opening.
Sometimes such extreme reactions are addressed by the delay time DLY_TM and the ISA opening decrease pattern. But in these instances, it has been found that there is a loss of fuel economy and/or driving performance. Furthermore, an empirically determined delay time DLY_TM and ISA opening decrease pattern is unique for each engine. Therefore, they change when specifications of the engine change, necessitating a lot of investigation for each engine.